Tài liệu Lịch khai giảng trong các hệ thống thời gian thực P9 pptx

Production management computerControl computer of the input silo for the reels Product control computer Dig computer Thickness variation control Mod computer Optic fibre network providin

Case Studies

9.1 Real-Time Acquisition and Analysis

of Rolling Mill Signals

9.1.1 Aluminium rolling mill

Manufacturing process of an aluminium reel

The P´echiney Rh´enalu plant processes aluminium intended for the packaging market.The manufacturing process of an aluminium reel is made up of five main stages:

1 The founding eliminates scraps and impurities through heat and chemicalprocesses, and prepares aluminium beds of 4 m× 6 m × 0.6 m weighing 8–10 tons.

2 Hot rolling reduces the metal thickness by deformation and annealing and forms a bed into a metal belt 2.5–8 mm thick and wound on a reel

trans-3 Cold rolling reduces the metal down to 250 micrometres (µm)

4 The thermal and mechanical completion process allows modification of the nical properties of the belt and cutting it to the customer’s order requirements

mecha-5 Varnishing consists of putting a coat of varnish on the belts sold for tins, foodpackaging or decoration

The packaging market (tinned beverages and food) requires sheets with a strict ness margin and demands flexibility from the manufacturing process Each rolling milltherefore has a signal acquisition and analysis system that allows real-time supervision

thick-of the manufacturing process

Cold rolling

Mill L12 is a cold rolling mill, single cage with four rollers, non-reversible, andkerosene lubricated Its function is to reduce the thickness of the incoming belt, whichmay be between 0.7 and 8 mm, and to produce an output belt between 0.25 and 4.5 mmthick, and with a maximum width of 2100 mm The minimum required thickness mar-gins are 5µm around the nominal output value The scheme of the rolling mill is given

in Figure 9.1

Active rollers (diameter: 450 mm) pulled by a d.c motor (2800 kW)

Cage

Winding roller

pulled by a d.c motor (1200 kW)

Direction of rolling stream

Belt thickness sensors

Figure 9.1 Scheme of the cold rolling mill

The thickness reduction is realized by the joint action of metal crushing between therollers and belt traction The belt output speed may reach 30 m/s (i.e 108 km/h) Therolling mill is driven by several computer-control systems which control the tighteninghydraulic jack and the motors driving the active rollers, the winding and unwindingrollers, the input thickness variation compensation, the output thickness control and thebelt tension regulation Three of the controlling computers share a common memory.Other functions are also present:

• production management, which prepares the list of products and displays it tothe operator;

• coordination of arriving products, initial setting of the rolling mill and preparation

of a production report;

• rolling mill regulation, which includes the cage setting, the insertion of the inputbelt, the speed increase, and the automatic stopping of the rolling mill;

Production management computer

Control computer

of the input silo for

the reels

Product control computer

(Dig computer)

Thickness variation control

(Mod computer)

Optic fibre network providing a shared memory

Real-time acquisition and analysis computer

Off-line processing computer

On-line display computer

Figure 9.2 Physical architecture of the rolling mill environment

• management of two silos, automatic stores where the input reels and the outputmanufactured reels are stored

Two human operators supervise the rolling mill input and output The physical ture of the whole application is given in Figure 9.2 where the production managementcomputer, the control computers and their common memory, and the signal acquisitionand analysis computer are displayed

architec-9.1.2 Real-time acquisition and analysis:

user requirements

Objectives of the signal acquisition and analysis system

The objectives of the rolling mill signal acquisition and analysis are:

• to improve knowledge of the mill’s behaviour and validate the proposedmodifications;

• to help find fault sources rapidly;

• to provide operators with a manufacture product tracing system

The signal source is the common memory of the three mill computers The acquisitionand analysis system realizes two operations:

• acquisition of signals which are generated by the rolling mill and their storage in

a real-time database (RTDB);

• recording of some user configured signals on-demand

Special constraints

The manufacturing process imposes availability and security constraints:

• Availability: the mill is operational day and night, with a solely preventive tenance break of 8 or 16 hours once a week

main-• Security: no perturbation should propagate up to the mill controlling systems sincethis may break the belt or cause fire in the mill (remember that the mill is lubricatedwith kerosene, which is highly flammable)

Signal acquisition frequency

The signal acquisition rate has to be equal to the signal production rate (which isitself fixed by the rolling evolution speed–the dynamics–and the Shannon theorem),and for the signal records to be usable, they have to hold all the successive acquiredvalues during the requested recording period The signals stored in the shared memorycome from:

• the Mod computer, which writes 984 bytes every 4 ms (246 Kbytes/s) and

addition-ally 160 bytes at a new product arrival (about once every 3 minutes);

• the Dig computer, which writes 544 bytes every 20 ms (27 Kbytes/s);

• the Pla computer, which writes 2052 bytes every 100 ms (20 Kbytes/s).

Rolling mill signal recording

It is required to record the real-time signal samples during a given period and after someconditioning The recorded signals must then be stored in files for off-line processing.The operator defines the starting and finishing times of each record and the nature ofthe recorded samples Records may be of three kinds:

• on operator request: for example when he wants to follow the manufacturing of aparticular product;

• perpetual: to provide a continuous manufacturing trace;

• disrupt analysis: to retrieve the signal samples some period before and after atriggering condition This condition may be belt tearing, fire or urgency stop.

The recording task has been configured to record 180 bytes every 4 ms over a 700 speriod and thus it uses files of 32 Mbytes These records are then processed off-line,without real-time constraints

Immediate signal conditioning

The immediate signal conditioning includes raw signal analysis, real-time evolutiondisplay and dashboard presentation

1 The raw signal analysis provides:

– statistical information about a product and its quality trends;

– computation of the belt length;

– filtering treatment of the signal to delete noise and keep only the useful part

of the signal, i.e the thickness variations around zero

2 Some values are displayed in real-time:

– thickness variations of the input and output belt, with horizontal lines to pointout the acceptable minimum and maximum;

– flatness variations of the input and output belt This flatness evolves duringthe production since heat dilates the rollers Flatness is depicted on a coloureddisplay called the flatness cartography To get this cartography, the belt thick-ness is measured by 27 sensors spread across the belt width and is coded by acolour function of the measured value The belt is plane when all the measureshave the same colour This allows easy visualization of the flatness variations

3 The dashboard displays these evolutions, some numerical values, information anderror messages, belt flatness instructions, and manufacturing characteristics (alloy,width, input and output nominal thickness, etc.) The screen resolution and its

The belt applies different pressures on the roller

The roller generates different pressures on the sensors according to the applied force.

Each sensor measurement is coded by a colour function and the set of sensors provides

a flatness cartography

Coded sensor values at time t Coded sensor values at time t + 1 Coded sensor values at time t + 2 Coded sensor values at time t + 3 Coded sensor values at time t + 4

Belt flow

direction

This figure shows how the pressures are measured along a roller and how they are displayed as a flatness cartography.

Figure 9.3 Roller geometry and flatness cartography

renewal rate (200 ms) are adapted to the resolution and dynamics of the displayedsignals as well as to the eye’s perception ability

Automatic report generation

Every product passing in transit in the rolling mill automatically generates a report,which allows appraising of its manufacturing conditions and quality The reportedinformation is extracted from former computation and displays The report is prepared

in Postscript format and saved in a file The last 100 reports are stored in a circularbuffer before being printed The reports are printed on-line, on operator request orautomatically after a programmed condition occurrence The requirement is to be able

to print a report for every manufactured product whose manufacturing requires at least

5 minutes The report printing queue is scanned every 2 seconds

9.1.3 Assignment of operational functions to devices

Hardware architecture

The geographic distribution shows three sets:

• the control cabin for the operator, where the signal display and report printingfacilities must be available;

• the power station, where all signals should be available and where theacquisition and analysing functions are implemented (computation, recording,report generation);

• the terminal room, where the environment is quiet enough for off-line processing

of the stored records and for configuring the system

Hardware and physical architecture choices

The P´echiney Rh´enalu standards, the estimated numbers of interrupt levels andinput–output cards, and the evaluation of the required processing power led to thefollowing choices:

1 For the real-time acquisition and analysis computing system: real-time executiveLynxOs version 3.0, VME bus, Motorola 2600 card with Power PC 200 MHz,

96 Mbytes RAM memory, 100 Mbits/s Ethernet port and a SCSI 2 interface,

4 SCSI 2 hard disks, each with a 1 Mbyte cache memory, and 8 ms access time.With this configuration, LynxOs reports the following performance:

– context switch in 4 microseconds;

– interrupt handling in less than 11 microseconds;

– access time to a driver in 2 microseconds;

– semaphore operation in 2 microseconds;

– time provided by getimeofday()system call with an accuracy of 3 seconds

micro-2 For off-line processing and on-line display: two Pentium PCs

3 For connecting the real-time acquisition and analysis computer and the two otherfunctionally dependent PC computers: a fast 100 Mbytes CSMA/CD Ethernet withTCP/IP protocol

4 For acquiring the rolling mill data: the ultra fast optic fibre network Scramnet that

is already used by the mill control computers Scramnet uses a specific protocolsimulating a shared memory and allowing processors to write directly and read at

a given address in this simulated shared memory Each write operation may raise

an interrupt in the real-time acquisition and analysis computer and this interruptcan be used to synchronize it The data are written by the emitting processor inits Scramnet card The emission cost corresponds to writing at an address in theVME bus or in a Multibus, and the application can tolerate it The writing andreading times have been instrumented and are presented Table 9.1

Table 9.1 Scramnet access times

useful bytes

Mean time (µs)

Useful throughput (Kb/s)

9.1.4 Logical architecture and real-time tasks

Real-time database

The application shares a common data table that is used as a blackboard by allprograms, as shown in Figure 9.4 This table is resident in main memory and mappedinto the shared virtual memory of the Posix tasks Data are stored as arrays in the table

To allow users to reference the signals by alphanumeric names, as well as ing tasks to access them rapidly by addresses in main memory, dynamic binding isused and the binding values are initialized anew at each database restructuring Thisuse of precompiled alphanumeric requests causes this table to be called a real-timedatabase (RTDB)

allow-Real-time tasks

The set of periodic tasks and the recording of the rolling steps (rolling start, eration, rolling at constant speed, deceleration, rolling end) are synchronized by the

accel-emission of the Mod computer signals every 4 ms This fastest sampling rate fixes the

basic cycle In the following we present the tasks, the precedence relations betweensome of them, the empirically chosen priorities, and the task synchronization imple-mentation The schemas of some tasks are given in Figures 9.5 and 9.8

The three acquisition tasks: modcomp, digigage and planicim The acquisition ofrolling mill signals must be done at the rate of the emitting computer This hard

Read or write access Task symbol

demand processing

Acquisition tasks

modcomp, digigage, planicim

Figure 9.4 Real-time database utilization

Rolling mill signal acquisition and RTDB writing Acquisition tasks

Reading signals from RTDB and copying them in a buffer (producer) Archiving task

Read buffer (consumer) Disk writing Recording task

Real-time database RTDB

Input–output buffer

Disk file

Figure 9.5 The recorded data flow

timing constraint (due to signal acquisition frequency) is necessary for recording the

rolling mill dynamics correctly Flatness regulation signals come from the Pla puter with a period of 100 ms Thickness low regulation signals come from the Dig

com-computer with a period of 20 ms Thickness rapid regulation signals are issued from

the Mod computer with a period of 4 ms One acquisition task is devoted to each of

these signal sources An interrupt signalling the end of writes in Scramnet is set by the

writer We note the three acquisition tasks as modcomp, digigage and planicim The

acquisition task deposits the acquired signals in the RTDB memory-resident database.The interrupt signal allows checking whether the current computation time of a taskremains lower than its period A trespassing task, i.e one causing a timing fault, is setfaulty and stopped This also causes the whole acquisition and analysis system to stop,without any perturbation of the rolling mill control or the product manufacturing

Activation conditions task: cond activ The activation condition task (called

cond activ ) is the dynamic interpreter of the logic equations set specifying the list

of samples to record or causing automatic recording to start when the signals detectthat a product has gone out of tolerance These logic equations are captured at systemconfiguration, parsed and compiled into an evaluation binary tree This task is triggered

every 4 ms by the modcomp task with a relative deadline value equal to its period.

Immediate signal processing task: processing The signal processing task (called

pro-cessing) reads the new signal samples in the database, computes the data to be displayed

or stored and writes them in the database It computes the statistical data, the FFT,the belt length, and the filtering of some signals This processing must be done at theacquisition rate of the fastest signals to recording the rolling mill dynamics correctly

This task is triggered every 4 ms by the modcomp task with a relative deadline value

equal to its period

Record archiving tasks: storage, perturbo and demand The three record archiving

tasks, called storage, perturbo and demand, must operate at the acquisition rate of the

fastest signals This means that some timing constraints have to be taken into account

to record the rolling mill dynamics correctly Thus the tasks are released every 4 ms

by the modcomp task with a relative deadline value equal to its period Each task

reads the recorded signals in the database and transfers them to files on disks, usingproducer–consumer schemes with a two-slot buffer for each file The archiving tasks

(i.e storage, perturbo and demand tasks) write to the buffers while additional tasks, called recording consume from the buffers the data to be transferred to disks Those

recording tasks, one per archiving task, consume very little processor time and this

can be neglected They have a priority lower than the least priority task of period 4 ms(their priority is set to 5 units below their corresponding archiving task)

Signal displaying task: displaying Signal displaying (task called displaying) requires

a renewal rate of 200 ms This is a deadline with a soft timing constraint, since anydata which is not displayed at a given period may be stored and displayed at the nextperiod There is no information loss for the user, who is concerned with manufacturing

a product according to fixed specifications For this he or she needs to observe theminimum, maximum and mean values of the signal since the last screen refresh Thedisplay programs use an X11 graphical library and the real-time task uses the PC as

an X server

Report generating task: reporting The reports must be produced (by the task called

reporting) with a period of 200 ms This task also has a soft deadline.

Report printing task: printing Report printing (the task is named printing) is required

either automatically or by the operator The task is triggered periodically every twoseconds and it checks the Postscript circular buffer for new reports to print

Initializing task: starting The application initialization is an aperiodic task (called

starting) which prepares all the resources required by the other tasks It is the first to

run and executes alone before it releases the other tasks A configuration file specifiesthe number, type and size of files to create There may be up to 525 files, totalling2.5 Gbytes All files are created in advance, and are allocated to tasks on demand Atthe first system installation, this file creation may take up to one hour

Closing task: termination The application closure is performed by an aperiodic

task (called termination) which releases all used resources It is triggered at the

application end

Precedence relationships

The successive signal conditionings involve precedence relationships between the tasks:acquisition must be done before signal processing and the evaluation of activationconditions These tasks must in turn precede record archiving, display and report gen-

eration Starting precedes every task and termination stops them all before releasing

their resources Figure 9.6 shows the precedence graph

When the task modcomp has set the signal samples in the database, it activates the other periodic tasks which use these samples; digigage and planicim, which have larger

periods, also deposit some samples The 4 ms period tasks check a version number toknow when the larger period samples have been refreshed

Simple precedence relationship (after one execution of τ1 there are

n executions of τ2)

modcomp

1/5

1/25

τ1 τ2 τ1 precedes τ2 once every n

Figure 9.6 Tasks precedence graph

Empirical priorities of tasks

The LynxOs system has a fixed priority scheduler, with 255 priority levels, the higherlevel being 255 The priorities have been chosen on a supposed urgency basis and thehigher priorities have been given to the tasks with the harder timing constraints It hasbeen checked that the result was a feasible schedule

Table 9.2 presents the empirical constant priorities given to each task, the period T , the measured computation time C (the minimum, maximum and mean values have

been recorded by measuring the start and finish time of the requests with the

getimeofday()system call), the relative deadline D and the reaction category in

case of timing fault

Synchronization by semaphores

In the studied system, the periodic tasks are not released by a real-time schedulerusing the system clock The basic rate is given directly by the rolling mill and by the

end-of-write interrupt which is generated every 4 ms by the Mod computer.

The task requests triggering and the task precedence relationships are programmed

with semaphores which are used as synchronization events Recall that a semaphore S

is used by means of two primitives, P (S) and V (S) (Silberschatz and Galvin, 1998;

Tanenbaum and Woodhull 1997)

Table 9.2 The tasks of the acquisition and analysis system

ing when executing P(S processing), demand when executing P(S demand), and so

on At each 4 ms period end, all the tasks are blocked when there is no timing fault

The Mod computer end-of-write interrupt causes the execution of a V(S modcomp) operation, which awakes the modcomp task When this task finishes and just before blocking again by executing P(S modcomp), it wakes up all the other periodic tasks

by executing V(S cond activ), V(S processing), , V(S demand) Every 5 cycles it wakes task digigage; every 25 cycles it wakes task planicim; ; every 500 cycles it wakes task printing The execution order is fixed by the task priority (there is only one processor and the cyclic tasks are not preempted for file output since the recording

tasks have lower priorities) This implements the task precedence relationships Thesynchronization of the 11 cyclic tasks is depicted in Figure 9.7

Task modcomp also monitors each taskτx it awakes In nominal behaviour, τx is

blocked at the time of its release This is checked by modcomp reading S τx’s state

(S τx is the private semaphore ofτx ) S τx’s state represents the history of operations

on S τx and it memorizes therefore whether, before being preempted by modcomp,

the cyclic taskτx was able or not to execute the P (S τx )operation which ends thecycle, blockingτx anew This solution is correct only in a uniprocessor computer and

if modcomp is the highest priority task and able to preempt the other tasks.

To sum up, the task modcomp starts each 4 ms cycle when receiving the Scramnet interrupt mapped to a V semaphore operation It executes its cyclic program, checks

the time limit of the tasks and then awakes all the tasks concerned with the current

cycle Figure 9.8 presents the task schema of modcomp and of the archiving tasks Finally, when a task needs signals acquired by a task other than modcomp, it reads

the database and checks for them Each of the data structures resulting from acquisition

or processing is given a version number that is incremented at each update The clientprograms have their own counter and compare its value to the current version number

to check for a new value The version numbers are monotonously increasing If their

Mod interrupt V(S_modcomp)

to use a mutual exclusion semaphore

Reactions to timing faults

Timing faults are detected by task modcomp as explained above The reaction depends

on the criticality of the faulty task (Table 9.2) and is related to one of the ing categories:

follow-• Category 1: the computing system is stopped since the sampled signals do not sent the rolling mill dynamics The values have not been read at the same sampling

repre-instant (this category concerns the three acquisition tasks, modcomp, digigage and

planicim).

Archiving task/** tasks storage, perturbo and demand

begin

open database

open synchronization table

open allocation table

start the buffer consumer task

while (no required stop) loop

wait for a required archive

read configuration and open archiving file

create the two slots buffer for the recorded signals

/** each buffer size is set to the recorded signal size and rate

wait for the start recording authorization /** blocked by P(S_producer)

while (not(end recording condition) or not(max recording time))loop

write each signal in its current buffer

if the current buffer is full then

activate the consumer recording task /** with V(S_consumer)

point to the other buffer /** with P(S_producer)

end if

end loop

wait until the last buffer is saved

close the archiving file

end loop /** loop controlled by (no required stop)

close database

close synchronization table

close allocation table

end/** archiving task

Recording task

begin

while (no required stop) loop

wait until a buffer is ready /** with P(S_Consumer)

transfer the buffer to the file, indicate free buffer /** with V(S_producer)

end loop /** loop controlled by (no required stop)

end/** Recording task

Acquisition task /** task modcomp

begin

Scramnet initialization

open database

open synchronization table

while (no required stop)loop

wait the Scramnet interrupt /** with P(S_modcomp)

read Scramnet and write the samples in the database

monitor other tasks

awake the other tasks, tx /** with V(S_tx)

end loop /** loop controlled by (no required stop) close database

close synchronization table

end/** acquisition task

Figure 9.8 Modcomp and archiving task schemes

• Category 2: the computing system is stopped since the computed values areincorrect and useless (this category concerns the conditions elaboration task,

cond activ , and the signal processing task: processing).

• Category 3: the function currently performed by the task is stopped since its results

are not usable (this category concerns the three record archiving tasks, storage,

perturbo and demand ).

• Category 4: the current function is not stopped but the fault is recorded in thelogbook (journal) The recurrent appearance of this fault may motivate the oper-ator to alleviate the processor load by augmenting the task period or reducingthe number of required computations (this category concerns the signal displaying

task, displaying, the report generating task, reporting, and the report printing task,

• Study the schedulability of the 11 periodic tasks with an on-line empirical fixedpriority scheduler as in the case study

• Study the schedulability of the 11 periodic tasks with the RM algorithm

• Study the schedulability of the 11 periodic tasks with the EDF algorithm

Scheduling with shared exclusive resources

Let us suppose that the shared data in the database are protected by locks implemented

with mutual exclusion semaphores (P or V operation time is equal to 2 microseconds).

Analyse the influence of access conflicts, context switches (the thread context switchtime is equal to 4 microseconds) and the additional delays caused by the databaselocking with different lock granularity

Robustness of the application

Compute the laxity of each task and the system laxity for:

• evaluating the global robustness For example, consider slowing down the processorspeed as much as acceptable for the timing constraints

• evaluating the margin for the task behaviour when its execution time increases

• estimating the influence of random perturbations caused by shared resource locking

To introduce some timing jitter, it is necessary to increase the processor utilizationfactor of some tasks Reducing the period of some tasks can do this, for example.Then, once a jitter has appeared:

• introduce a start time jitter control for the signal displaying task,

• introduce a finish time jitter control for the processing and reporting tasks This

allows simulating a sampled data control loop monitoring the actuators

Multiprocessor architecture

Let us suppose a multiprocessor is used to increase the computing power Study thetask scheduling with two implementation choices In the first one, the basic rate is stillgiven by the rolling mill, and cyclic task synchronization and wake up are done byprogram In the second case, the LynxOs real-time clock (accuracy of 3 microseconds)and a real-time scheduler are used

Task precedence must be respected and the mixing of priorities and event-likesemaphores cannot be used, since the uniprocessor solution is no longer valid Thefault detection that the redundancy allowed is not valid either

Network

The use of Scramnet is costly Examine the possibilities and limits of other time networks and other real-time protocols Consider several message communica-tion schemes between the application tasks Finally, as in the example presented inSection 6.4.3, consider message scheduling when the network used is CAN, FIP or atoken bus

real-9.2 Embedded Real-Time Application: Mars

Pathfinder Mission

9.2.1 Mars Pathfinder mission

After the success of early Mars discovery missions (Viking in 1976), a long series ofmission failures have limited Mars exploration The Mars Pathfinder mission was an

important step in NASA discovery missions The spacecraft was designed, built andoperated by the Jet Propulsion Laboratory (JPL) for NASA Launched on 4 December

1996, Pathfinder reached Mars on 4 July 1997, directly entering the planet’s atmosphereand bouncing on inflated airbags as a technology demonstration of a new way to deliver

a lander of 264 kg on Mars After a while, the Pathfinder stationary lander released amicro-rover, named Sojourner The rover Sojourner, weighing 10.5 kg, is a six-wheeledvehicle controlled by an earth-based operator, who used images obtained by both therover and lander systems This control is possible thanks to two communication devices:one between the lander and Earth and the other between the lander and the rover, done

by means of high frequency radio waves The Mars Pathfinder’s rover rolled onto thesurface of Mars on 6 July at a maximum speed of 24 m/h Sojourner’s mobility providedthe capability of discovering a landing area over hundreds of square metres on Mars.The scientific objectives included long-range and close-up surface imaging, and,more generally, characterization of the Martian environment for further exploration.The Pathfinder mission investigated the surface of Mars with several instruments:cameras, spectrometers, atmospheric structure instruments and meteorology, known

as ASI/MET, etc These instruments allowed investigations of the geology and face morphology at sub-metre to one hundred metres scale During the total mission,the spacecraft relayed 2.3 gigabits of data to Earth This huge volume of informationincluded 16 500 images from the lander camera and 550 images from the rover cam-era, 16 chemical analyses and 8.5 million measurements of atmospheric conditions,temperature and wind

sur-After a few days, not long after Pathfinder started gathering meteorological data, thespacecraft began experiencing total resets, each resulting in losses of data By using anon-line debug, the software engineers were able to reproduce the failure, which turnedout to be a case of priority inversion in a concurrent execution context Once theyhad understood the problem and fixed it, the onboard software was modified and themission resumed its activity with complete success The lander and the rover operatedlonger than their design lifetimes We now examine what really happened on Mars tothe rover Sojourner

9.2.2 Hardware architecture

The simplified view of the Mars Pathfinder hardware architecture looks like the processor architecture, based on the RS 6000 microprocessor, presented in Figure 9.9.The hardware on the rover includes an Intel 8085 microprocessor which is dedi-cated to particular automatic controls But we do not take into account this processorbecause it has a separate activity that does not interfere with the general control ofthe spacecraft

one-The main processor on the lander part is plugged on a VME bus which also containsinterface cards for the radio to Earth, the lander camera and an interface to a specific

1553 bus The 1553 bus connects the two parts of the spacecraft (stationary lander androver) by means of a high frequency communication link This communication linkwas inherited from the Cassini spacecraft Through the 1553 bus, the hardware on thelander part provides an interface to accelerometers, a radar altimeter, and an instrumentfor meteorological measurements, called ASI/MET